This invention relates generally to heads for extruders. More particularly, this invention is concerned with a head for extruding hollow thick-walled cylindrical articles of indefinite length from plasticized synthetic resinous materials.
During the extrusion of hollow cylindrical articles of indefinite length, such as pipe, weld lines frequently occur in the finished article. Such weld lines are not indications of welding in the classical sense of the word, but result from the flow of plasticized polymeric material around streamline bodies in conventional spider pipeheads. The streamline bodies radially cut the material flow and have a tendency to align polymeric chains in the direction of material flow. As the flow recombines downstream of each streamline body, adhesion must be obtained between radially aligned surfaces.
Many polymeric materials do not readily adhere and recombine after having been cut. This property in combination with partially aligned polymeric chains results in non-isotropic structural properties of a finished article in addition to visual imperfections in the surface thereof.
Many other polymeric materials do readily adhere and recombine after being cut. However, for these materials the weld lines are actually visual imperfections in the finished article and, in many applications, it is highly desirable to avoid and eliminate the presence of such visual imperfections.
Another problem, frequently encountered in the extrusion of large diameter pipe, concerns the physical mass and weight of the pipehead assembly. Conventionally, spider designs are used for extrusion of large diameter pipes. As a result of high extrusion pressures in the plasticized material and the large surface areas internally of the pipehead, substantial forces are acting to separate parts of the pipehead. These forces necessitate large and heavy parts to withstand the fluid pressure developed during the extrusion process.
In the past it has been proposed to design a pipehead so as to overcome the problems usually associated with the conventional spider designs. One such pipehead, illustrated by U.S. Pat. No. 3,270,371 to Schiedrum et al., includes a confined helical channel of constant cross-sectional area that spirals radially outwardly to an annular distributing channel from which a pipe is extruded. However, this proposal is believed to be of limited practical utility as far as commercial operations are concerned.
In commercial applications, tolerance to variations in throughput rate, the particular polymeric material used and melt temperature is necessary, since pipes of different materials are often extruded with the same pipehead. Also, in most commercial applications, it is desirable and frequently a practical necessity to have a finished product with isotropic structural properties. Moreover, since polymeric materials exhibit poor pressure transmitting features, pipeheads which supply a plenum by a single channel frequently exhibit a poor circumferential pressure distribution at the extrusion orifice.
The general problem of transforming a solid cylindrical flow of plastic material into a uniform annular flow has also been the subject of various proposals in the field of blowing plastic films. See, for example, U.S. Pat. No. 3,689,192 to Upmeier, where a horizontal stream entering the periphery of a vertically oriented blowhead is split into circumferential streams which enter helical channels with decreasing depth. Aside from the basic unsuitability of the vertical blowhead devices for use in the extrusion of pipe, this construction is believed incapable of accommodating the variables necessarily incident to practical pipe making operations.
Where material may flow from an upstream portion of one channel over a downstream portion of another channel, a radially laminated wall construction occurs which may be free from weld lines. However, if the throughput rate, temperature or polymeric material, varies, the helical channel empty earlier or later thereby developing a weld line which corresponds to the end of the channel. Moreover, where material discharged from a few channels is promptly discharged from an extrusion orifice, a pressure maldistribution is likely to occur.